Best to rule out battery issues first surely. The fact that video works does NOT rule it out! Can't explain it, but on mine at least a battery swap cured it instantly.
Just order a cheap DSTE (or similar clone) battery and charger - cost me just £11.99 (IIRC, in UK) for two batteries and a charger via ebay. Then if the camera works with the new battery (as mine did), it is the old battery that is the problem. The old battery didn't work (i.e. same prob - shut down on pics, OK on vids) even when charged on the DTSE charger, and the DTSE batteries worked fine after being topped up by the original Pentax charger, so a problem with the original Pentax charger was also ruled out.
Ah - just thought I'd best check - looks like the price for DTSE batteries has gone up a bit (or it was listed super cheap when i got mine!). Now £16.50 on UK ebay for two batteries plus charger: e.g.
DSTE 2pcs D-LI90 Battery + Charger for Pentax D-LI90 K3 K5 K7 645D Camera new | eBay
Still not bad IMO, sure similar deals are available on US Ebay and elsewhere.
Getting the new batteries and charger meant I could definitely locate the problem (so no quibbles at all from the supplier about replacing the original battery), didn't have to send the cam away (so could start using it again as soon as the new batteries arrived) and I would have wanted spare batteries anyway. Not done any proper testing but so far the cheap clone batteries seem to be about as good as the replacement Pentax battery which eventually arrived after the warranty return. Never had the problem re-appear since.
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